Pressure sensors serve for registering pressures and are applied, for example, in pressure measuring devices, which are used in industrial measurements technology.
In pressure measuring technology, popularly applied as pressure sensors are so-called semiconductor sensors, e.g. silicon chips with doped, piezoresistive, resistance elements. Semiconductor sensors of this type include, arranged on a carrier, a measuring membrane, or diaphragm, whose one side is exposed in measurement operation to a pressure to be measured. Pressure sensor chips are, as a rule, very sensitive and are therefore not directly exposed to a medium, whose pressure is to be recorded. Instead, a liquid filled, pressure transfer means is interposed. Pressure acting on the measuring membrane effects a pressure-dependent deflection of the measuring membrane, which is registered by means of the doped resistance elements and converted into an electrical signal, which is then available for additional processing and/or evaluation.
Semiconductor sensors are today regularly produced using silicon, e.g. by use of Silicon On Insulator (SOI) technology. In such case, preferably BESOI (Bonded and Etched-back Silicon On Insulator) wafers are used as starting material. BESOI wafers are produced by means of “silicon direct bonding”. For this, two oxidized silicon wafers are placed together and bonded under pressure and high temperature. In this way, a three layer wafer is obtained, wherein, between two silicon layers, an oxide layer is located. The buried oxide layer, referred to with the acronym BOX (Buried OXide layer), has a thickness of a few nm up to a few μm. This composite is thinned and polished from one side. The thinned, polished side becomes the active layer. The active layer can be a few μm thick and is referred to in the English language literature as e.g. device wafer or Silicon OverLayer (SOL). The thickness of the active layer can be produced with today's manufacturing processes very exactly and uniformly and with high reproducibility.
An essential advantage of the application of BESOI wafers for the manufacture of pressure sensors is that the buried oxide layer (BOX) forms a reliable etch stop. This is utilized, above all, for the manufacture of movable electrodes of capacitive pressure sensors. There are, however, also methods known, in the case of which, BESOI wafers are applied for the manufacture of piezoresistive pressure sensors. Such a method is described in the article: “Optimized technology for the fabrication of piezoresistive pressure sensors”, by A. Merlos, J. Santander, M. D. Alvares and F. Campabadal, published in the year 2000 in the Journal of Mechanical Engineering, Vol. 10, pages 204 to 208. There, it was shown, that one can produce, with BESOI wafers, sensor chips, which have an exactly defined membrane thickness, which is constant over the entire region of the membrane. Especially, piezoresistive sensors can be produced with very thin membranes with smallest of thickness tolerances of less than 1 μm. For this, there is etched into the silicon layer lying opposite to the active layer a cavity, via which the membrane is exposed. In such case, the buried oxide layer serves as etch stop. The outer edge of the silicon layer remaining after the etching procedure borders the cavity externally and forms a carrier for the membrane exposed through the cavity. Following this etching procedure, the region of the oxide layer serving as etch stop is removed by an additional etching procedure.
The sensitivity of piezoresistive pressure sensors is inversely dependent on the thickness of the membrane. The thinner the membrane, the more sensitive is the sensor. Unfortunately, however, also non-linearity of the sensors rises as membranes become thinner. The electrical measurement signal read from the doped resistances rises in the ideal case linearly with the pressure acting on the membrane. The thinner the membrane is, the greater are the deviations from this desired, linear relationship. This leads, especially in the case of pressure sensors for measuring smaller pressures, which need thin membranes, to measurement errors, or to a high effort for the compensation of such measurement errors.
A solution for this problem is to equip the membrane with a bending-stiff center. Such a bending-stiff center is produced, for example, on the rear side of the membrane facing away from the active layer by suitable working of the silicon to provide there a centrally arranged, silicon pedestal. The height of the pedestal is slightly smaller than the height of the membrane carrier surrounding the exterior of the pedestal. In this way, it is assured, that the pedestal is supported exclusively by the membrane, and, especially, that it does not lie against a holder, on which the membrane carrier is supported. Pedestal and membrane carrier are separated from one another by a cylindrical gap surrounding the pedestal, through which an outer edge of the membrane is exposed. In this way, a higher ratio of sensitivity to non-linearity is achievable. However, a bending-stiff center means that the membrane is thereby, on the whole, stiffer and, therewith, insensitive. The increasing of the ratio means a reducing of the sensitivity of the pressure sensor, which, especially in the case of measuring smaller pressures, is disadvantageous. Often, a compromise must be accepted in such case.
In US-A 2005/0274191, a method is described, with which the sensitivity of piezoresistive pressure sensors can be increased. For this, a part of the active layer in the immediate environment of the doped piezoresistive elements is etched away in such a manner, that the elements are surrounded by a furrow. The depth of the furrows is equal in such case to about the thickness of the doped resistances. The furrows act as stress concentrators and effect, therewith, an increasing of the sensitivity of the sensors, without requiring that the membrane must be enlarged, or its thickness reduced over a large amount of its total surface area.
This method is burdened, however, by the disadvantage, that temperature dependent stress states can occur on the front side of the membrane exposed in measurement operation to the pressure to be measured. Such stress states are attributable to the different thermal coefficients of expansion of silicon and the buried oxide layer and the different geometries of the two membrane sides. These stress states are temperature-dependent and lead to measurement errors. This effect arises especially strongly, since these thermal stress states occur in the immediate vicinity of the doped piezoresistive elements and these elements react very sensitively to stresses.